Natural gas typically contains some amount of moisture. In many applications, the removal or significant reduction of moisture, i.e., gas dehydration, is important. While several methods exist for gas dehydration, including Pressure Swing Absorption (PSA), Temperature Swing Absorption (TSA), solid desiccants (silica gel, molecular sieves or activated alumina) and cryogenics (including Joule-Thompson Expansion, with or without glycol), glycol dehydration is the most commonly used technology for natural gas drying. Glycol dehydration is a liquid desiccant system for the removal of water from natural gas and natural gas liquids (NGL). It is the most common and economical means of water removal from these streams. Glycols typically seen in industry include triethylene glycol (TEG, BP=285° C.), diethyleneglycol (DEG, BP=245° C.), monoethyleneglycol (MEG, BP=197° C.), and tetraethylene glycol (TREG, BP=314° C.). TEG is the most commonly used glycol in industry for natural gas dehydration.
In a typical glycol dehydration system, lean, water-free glycol (purity >99%) is fed to the top of an absorber (also known as a “glycol contactor”) where it is contacted with the wet natural gas stream. The glycol removes water from the natural gas by physical absorption and is carried out the bottom of the column. Upon exiting the absorber the glycol stream is often referred to as “rich glycol”. The dry natural gas leaves the top of the absorption column and is fed either to a pipeline system or to a gas plant. Glycol absorbers can be either tray columns or packed columns. After leaving the absorber, the rich glycol is fed to a flash vessel where hydrocarbon vapors are removed and any liquid hydrocarbons are skimmed from the glycol. This step is necessary as the absorber is typically operated at high pressure and the pressure must be reduced before the regeneration step. Due to the composition of the rich glycol, a vapor phase having a high hydrocarbon content will form when the pressure is lowered.
After leaving the flash vessel, the rich glycol is heated in a cross-exchanger and fed to the stripper (also known as a regenerator). The glycol stripper consists of a column, an overhead condenser, and a reboiler. The glycol is thermally regenerated to remove excess water and regain the high glycol purity. The hot, lean glycol is cooled by cross-exchange with rich glycol entering the stripper. It is then fed to a lean pump where its pressure is elevated to that of the glycol absorber. The lean solvent is cooled again with a trim cooler before being fed back into the absorber. This trim cooler can either be a cross-exchanger with the dry gas leaving the absorber or an air-cooled exchanger.
Most glycol units are fairly uniform except for the regeneration step. Several methods are used to enhance the stripping of the glycol to higher purities (higher purities are required for dryer gas out of the absorber). Given that the reboiler temperature is limited to 400 F or less to prevent thermal degradation of the glycol, almost all of the enhanced systems center on lowering the partial pressure of water in the system to increase stripping. Common enhanced methods include the use of stripping gas, the use of a vacuum system (lowering the entire stripper pressure), the DRIZO process, which is similar to the use of stripping gas but uses a recoverable hydrocarbon solvent, and the Coldfinger process where the vapors in the reboiler are partially condensed and drawn out separately from the bulk liquid. Glycols are cheaper, as compared to solid desiccants: 50% less at flows of 10 MMSCFD and 53% less at 50 MMSCFD, while the pressure drop is only 5-10 psig, in comparison to 10-50 psig for solid desiccants (silica gel, molecular sieves or activated alumina). Glycols also require less heat for regeneration per pound of water removed. Glycol circulation rates are typically 2-3 gallons/lb of H2O removed. Once saturated with water, the glycols are heated to 100° C. to boil off the water in a separate loop, to regenerate the glycols for repeated use for water absorption. The energy requirements for such a system are quite high, due to the high heat of vaporization of water.
Polyethylene glycol (PEG) is an oligomer or polymer of ethylene glycol, and very soluble in water. The EO monomer exhibits polarity of the individual C—O bonds in the molecule, allowing for interaction with polar groups. Thus, the ethylene oxide (EO) monomer in a polyethylene glycol enables very easy hydrogen bonding with water molecules, resulting in the very high osmotic pressures exhibited by PEGs. The EO monomers in the glycol chain have very high hydrophilicity, and has also been used as osmotic agents in the desalination and bio-medical industries. Each EO monomer has a capacity for physically absorbing almost 2.75 molecules of water.